Lithographic masks have a pattern of features that are desired to be imparted into a workpiece, in order to fabricate, for example, semiconductor integrated circuits or other microminiature devices in the workpiece. Typically, the features of the mask are transferred to the workpiece by first placing the mask on the object plane of an optical imaging system that focuses optical radiation propagating through the mask, in order to transfer the features of the mask to a photoresist layer either overlying the workpiece or overlying a layer of material which overlies the workpiece. Then the photoresist layer is developed in order to form a patterned photoresist layer. Thus, the pattern of the mask features is transferred to the photoresist layer, with a magnification (greater to, equal to, or less than unity) that depends upon the design of the optical imaging system. The resulting pattern of the photoresist features is used to define features in the workpiece--such as edges of ion implant regions, edges of removed portions of the layer(s) of material overlying the workpiece to be removed, or edges of portions of the workpiece itself--all these edges thus being in accordance with the edges of the mask features.
A limiting factor in thus transferring the mask features to the photoresist layer is diffraction caused by the non-vanishing wavelength .lambda. (in the vacuum) of the optical radiation that is used in the optical imaging system. As a result of this diffraction, the edges of the images formed by the optical focusing system become less sharp than desired, whereby the resolution of the mask features when focused on the photoresist layer deteriorates. To improve this resolution, phase-shifting masks have been proposed. In these masks, the phases of the optical radiation propagating through the mask are shifted by, say, .phi. radian in some regions relative to others. For example, a typical portion 9 (FIG. 1) of a prior-art phase-shifting mask (hereinafter, "mask portion 9") has an opaque chrome (chromium) layer pair 13 located on a portion of the top substantially planar surface 14 of a raised (plateau) region 11 of a transparent, typically quartz, substrate 10. The region 11 is thicker, by a distance h, than an adjacent "trench" region having a width w. This trench region has a top surface 12 which is substantially planar except for the presence of excess material--typically quartz--forming an undesired defect region 21. As used herein, the term "substantially planar" refers to a situation in which any departures from planarity do not degrade, by more than desired amounts, the sharpness of images produced (e.g., in the photoresist layer) when the mask portion 9 is used in the optical imaging system.
The distance h corresponds to the optical phase shift .phi. (radian). Thus, (hn)/.lambda.-h.lambda.=.phi., or h=.phi..lambda./(n-1), where n is the refractive index of the quartz. Typically, .phi.=.pi. radian; so that typically, h=.pi..lambda./(n-1). When viewed from above, the contours of the chrome layer pair 13 depend upon the contours of the ultimately desired features in the workpiece (or in a layer overlying the workpiece).
One problem that can arise in using the phase-shifting mask portion 9 is the presence of the above-mentioned defect region 21. It should be understood that, in general, the height of the defect region 21 can vary from point to point, typically anywhere between h and a tiny fraction of h, or even zero (as indicated in FIG. 1). This defect region 21 is typically composed of the same material as that of the substrate 10, typically quartz, that was not, but should have been, removed during whatever earlier processing was used to produce the trench region. Such a defect region can also be located in the plateau region 11.
Obviously, when the mask portion 9 is used in the optical imaging system, the presence of the defect region 21 can deteriorate the image formed by the mask portion 9 on the photoresist layer located on the workpiece. Rather than discard entirely this mask 9 having such a defect region 21 (the mask being otherwise very valuable) it is desirable to have a method of repairing the mask, i.e., a method of removing the defect region 21 without damaging the rest of the phase-shifting mask 9 (not shown). Such defect regions could be removed by techniques involving a three-dimensional inspection of the mask--for example, by means of a scanning tunneling electron microscope--followed by ion-assisted milling or laser-assisted milling of the defect region. Such ion-assisted milling (without the three-dimensional inspection required for repairing phase masks) has been described, for example, in "Mask and Circuit Repair with Focused-Ion Beams," by T.D. Cambria and N. P. Economou, published in Solid State Technology, pp. 133-136, September 1987. However, such techniques require careful three-dimensional point-by-point control of the milling in accordance with the varying height, from point to point, of the defect region 21. It would therefore be desirable to have a method for removing the defect region without necessitating three-dimensional inspection of the mask or three-dimensional control of the removal process.